1. Field of the Invention
The invention relates to extrudable linear low density polyethylene (LLDPE) compositions and films produced therefrom which exhibit improved gas/vapor permeability. More specifically, the invention is directed to compositions and films comprised of LLDPE, a modified polyolefin and a polyester polyol. Additionally, the invention relates to a process of producing said compositions.
2. Description of the Prior Art
Breathable polyolefin films, i.e., films that are gas/vapor permeable, are increasingly in demand for medical and personal care applications. Polyolefin films, primarily polyethylene films, having increased water vapor transmission rates (WVTR) are commonly produced by incorporating high levels, e.g., 20-80 weight percent (wt. %) filler material, such as calcium carbonate, and monoaxially or biaxially stretching the film. By stretching the film after extrusion, voids are formed around the filler particles which permits diffusion through the film.
Breathable films of the above type are disclosed in U.S. Pat. No. 4,472,328. The reference discloses compounding the polyolefin resin, filler and liquid or waxy hydrocarbon elastomer such as hydroxyl-terminated liquid polybutadiene, to produce a porous film or sheet.
U.S. Pat. No. 4,350,655 discloses a porous polyolefin film containing at least 50 percent by weight of a surface coated inorganic filler. The inorganic fillers are surface coated with a fatty acid ester of silicon or titanium.
A problem with polymer films rendered breathable by incorporation of filler materials is that the incorporation of fillers also reduces the strength of the films. Reduction in tensile strength and elongation at break is disadvantageous and some references, such as U.S. Pat. No. 4,777,073, use specific combinations of resins and fillers and specific processing techniques in an effort to minimize these problems.
Still others utilize multi-layer constructions wherein a breathable core layer is combined with one or two skin layers. For example, U.S. Pat. No. 6,075,179 discloses films having a core layer formed from a mixture of thermoplastic resin and particulate filler coextruded with one or two adjacent support layers to increase strength. These support or skin layers account for a minor percentage of the total film thickness—typically less than 15%. A disadvantage of the skin layers is that they tend to reduce the moisture vapor breathability of the overall film since they block the pathways created by the pores in the core layer. The thicker the skin layer(s), the greater the reduction in breathability.
U.S. Pat. No. 6,682,803 seeks to overcome this problem by utilizing a mixture of two incompatible polymers for the skin layer(s). The incompatibility of the polymers results in the formation of cracks and/or tears when the laminate is stretched so that their presence does not significantly impair the moisture vapor breathability of the overall film.
There is a continuing need for polyolefin compositions which can be extruded into films having increased breathability and which are suitable for use as skin layers for porous core layers and for other applications. It would be even more advantageous if these compositions were produced using readily available commercial materials and if the resulting films, in addition to having increased WVTR and oxygen permeability, also exhibited a good balance of physical properties. These and other objectives are achieved with the compositions and films of the present invention.